UV-light stabilization additive package for solar cell module and laminated glass applications

ABSTRACT

An ultraviolet light stabilization additive package is used in an encapsulant material that may be used in solar cell modules, laminated glass and a variety of other applications. The ultraviolet light stabilization additive package comprises a first hindered amine light stabilizer and a second hindered amine light stabilizer. The first hindered amine light stabilizer provides thermal oxidative stabilization, and the second hindered amine light stabilizer providing photo-oxidative stabilization.

GOVERNMENT INTEREST

The subject matter described herein was supported in part byPhotovoltaic Manufacturing Technology (PVMaT) Contract No.ZAP-5-14271-09.

FIELD OF THE INVENTION

The invention relates to a UV light stabilization additive package forvarious applications. More particularly, the invention relates to a UVlight stabilization additive package for use in an encapsulant materialfor solar cell module and laminated glass applications.

BACKGROUND

Transparent encapsulant materials are used in numerous applications,including solar cell module and laminated glass applications. In solarcell applications, transparent encapsulants protect and seal theunderlying solar cells without adversely affecting the opticalproperties of such underlying materials. In laminated glassapplications, transparent encapsulants minimize any possible hazardsfrom broken glass. In these applications, the encapsulant is exposed tothe ultraviolet (UV) rays of the sun and this exposure can result in theyellowing and physical degradation of the polymer. To prevent this, UVstabilizers are added to the encapsulant.

In the manufacture of crystalline silicon solar cell modules, atransparent encapsulant material is used to protect the brittle siliconsolar cells from breakage and to help seal these cells into the overallmodule structure. The encapsulant material is usually a thermoplastic.The thermoplastic is melted, then flows to fill in any open spaces inthe module and bonds to all adjacent surfaces. The most widely usedencapsulant material for solar cell modules is a co-polymer of vinylacetate and ethylene, known as ethylene vinyl acetate (EVA). EVA is usedto encapsulate and seal both thin film and crystalline silicon solarcell modules.

There are several disadvantages associated with using EVA as anencapsulant material that adversely affect the quality and manufacturingcost of the solar cell modules. First, an organic peroxide is added toEVA in order to cross-link it using the heat which accompanies thelamination process. The cross-linking is necessary to increase the creepresistance of the encapsulated structure. However, the peroxide is notcompletely consumed during the cross-linking process, and the remainingperoxide can promote subsequent oxidation and degradation of EVA. Inaddition, the EVA must be laminated in a vacuum when making a modulebecause of the presence of peroxide in the EVA. The reason for this isthat oxygen lowers the degree of cross-linking, producing anunsatisfactory encapsulant. Second, the preferred EVA usually contains33% (by weight) of vinyl acetate, and thus is a very soft and tackysubstance that tends to stick to itself. This tackiness makes handlingof the EVA material in a manufacturing environment much moretroublesome. As such, the EVA material requires a release paper or linermaterial to use the material. Third, peroxide cured EVA has been knownto turn yellow and brown under extensive exposure to sunlight forseveral years. Yellowing and browning causes reduction in solar modulepower output. Fourth, EVA can produce acetic acid under processingconditions which can then foster metal contact corrosion. Fifth, EVA isknown to be fairly permeable to water and is, therefore, far from idealas a sealant.

Although virtually any transparent polymer eventually shows somedegradation and yellowing after exposure to sunlight, an encapsulantmaterial that can withstand degradation and yellowing for a longerperiod of time than EVA is desirable. Ideally, a solar cell moduleshould last for thirty years without showing much sign of degradation.EVA is unlikely to satisfy this thirty year duration requirement. Inaddition to finding a suitable replacement for EVA (or PVE, which isdescribed below), it is also necessary to develop a suitable UV lightstabilization package for the encapsulant.

In laminated glass applications, the laminated glass is made by forminga sandwich of two pieces of glass with a sheet of a transparent polymerdisposed between the two pieces. This transparent polymer sheet servesto prevent the glass in the laminated structure from shattering intodangerous shards when the glass is broken. Windshields on automobilesand architectural glass are manufactured in this manner. Poly vinylbutyral (PVB) is a widely used material in such polymer sheets in theforegoing laminated glass applications. PVB, however, has severaldrawbacks. First, PVB is extremely hydroscopic (i.e. it absorbs moisturereadily). Therefore, it must be kept refrigerated and maintained underspecial atmospheric conditions before it can be successfully laminated.Second, PVB is also extremely soft and tacky and, therefore, must beused with a release or liner layers.

SUMMARY OF THE INVENTION

This invention features an ultraviolet (UV) light stabilization additivepackage for use in an encapsulant material, which may be used in solarcell modules, laminated glass and a variety of other applications. TheUV light stabilization additive package includes a first hindered aminelight stabilizer and a second hindered amine light stabilizer. The firsthindered amine light stabilizer provides thermal oxidativestabilization, while the second hindered amine light stabilizer providesphoto-oxidative stabilization.

One aspect of the invention is that neither an ultraviolet absorber(UVA) nor an anti-oxidant (AO) is needed to provide the UV lightstabilization. Anti-oxidants are used to insure that the polymer doesnot oxidize excessively and thereby suffer degradation of itsproperties, while it is being formed into a sheet or a film. Experimentson an encapsulant material including the UV stabilization additivepackage indicate that the encapsulant material does not suffer any lossin physical properties with repeated extrusions even without theaddition of an anti-oxidant. The stabilization additive package of thepresent invention does not include ultraviolet absorbers, as they havebeen known to cause yellowing with extended exposure. Unlike the presentinvention, conventional formulations of EVA and PVB include bothanti-oxidants and ultraviolet absorbers.

In one embodiment, the encapsulant material comprises a three layerstructure. A middle layer is formed of metallocene polyethylene anddisposed between two outer layers of ionomer. The layer of metallocenepolyethylene can comprise co-polymers of ethylene with butene, hexene,or octene. The ionomer layers can be derived from any direct or graftedethylene co-polymer of an alpha olefin having the formula R—CH═CH₂,where R is a radical selected from the class consisting of hydrogen andalkyl radicals having from 1 to 8 carbon atoms and alpha,beta-ethylenically unsaturated carboxylic acid having from 3 to 8 carbonatoms.

In another embodiment, the UV light stabilization additive package isincorporated in an encapsulant surrounding a solar cell within a module.The solar cell module comprises at least one solar cell and atransparent encapsulant material disposed adjacent to at least onesurface of the solar cell. The encapsulant material comprises at least apolyethylene co-polymer and the ultraviolet light stabilization additivepackage. A front support layer formed of light transmitting material isdisposed adjacent a front surface of the encapsulant material, and abackskin layer is disposed adjacent a rear surface of the encapsulantmaterial. In one embodiment, the encapsulant material comprises a firstencapsulant material disposed adjacent a front surface of the solar celland a second encapsulant layer disposed adjacent a rear surface of thesolar cell.

In another embodiment, the ultraviolet light stabilization additivepackage is included in an encapsulant layer within a laminatedtransparent member. The laminated transparent member comprises a frontsupport layer, the encapsulant layer, and a rear support layer. Allthree layers are transparent. The transparent encapsulant layer isdisposed adjacent a rear surface of the front support layer. Theencapsulant layer comprises a polyethylene co-polymer and theultraviolet light stabilization additive package. The rear support layeris disposed adjacent a rear surface of the encapsulant layer.

The invention also features a method of manufacturing a solar cellmodule. According to the method, at least one solar cell is provided, atransparent encapsulant layer is formed and positioned adjacent at leastone surface of the solar cell, and the solar cell and the encapsulantmaterial are placed between a transparent front support layer and abackskin layer. The transparent encapsulant layer includes apolyethylene co-polymer and the ultraviolet light stabilization additivepackage.

In still another embodiment, the invention features a method ofmanufacturing a laminated transparent member. Two support layers formedof transparent materials are provided, and a transparent encapsulantlayer is formed and placed between the support layers to form anassembly. The assembly is laminated. The transparent encapsulant layerincludes a polyethylene co-polymer and the ultraviolet lightstabilization additive package.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an encapsulant materialincorporating the principles of the invention.

FIG. 2 is a cross-sectional view of a crystalline silicon solar cellmodule encapsulated with the encapsulant material of FIG. 1.

FIG. 3 is a cross-sectional view of a solar cell module in which theencapsulant material and a backskin encapsulates the solar cells.

FIG. 4 is a cross-sectional view of a Copper Indium Diselenide thin filmsolar cell module which includes the encapsulant material.

FIG. 5 is a cross-sectional view of an amorphous silicon solar cellmodule which includes the encapsulant material.

FIG. 6 is a cross-sectional view of a Cadmium Telluride thin film solarcell module which includes the encapsulant material.

FIG. 7 is a cross-sectional view of a laminated glass structure whichincludes the encapsulant material.

DETAILED DESCRIPTION

FIG. 1 is a cross-sectional view of a transparent encapsulant material10 incorporating the principles of the invention. In one embodiment, theencapsulant material 10 can be used in a solar cell module to protectcrystalline silicon solar cells from breakage when they are in a moduleand subjected to mechanical stress during field usage. The encapsulantmaterial also serves to seal the module, which is particularly importantfor thin film modules. In another embodiment, the encapsulant material10 can be laminated between two pieces of glass or transparent polymerto provide a composite structure that does not shatter when broken.

The encapsulant material 10 comprises an inner layer 12 and outer layers14 and 16. The outer layer 14 is disposed adjacent a front surface 18 ofthe inner layer, and the outer layer 16 is disposed adjacent a rearsurface 19 of the inner layer. The inner layer 12 comprises a highlytransparent thermoplastic material. The outer layers 14, 16 are made oftransparent polymer material that is capable of heat bonding to variousmaterials including glass, metals and other polymers.

In one embodiment, the inner layer 12 can be formed of metallocenepolyethylene and the outer layers 14, 16 can be formed of an ionomer.Metallocene polyethylene is prepared by using as a catalyst anorganometallic coordination compound which is obtained as acyclopentadienyl derivative of a transition metal or a metal halide. Theterm “ionomer” refers to a generic class of polymers which contain bothcovalent bonds and regions of ionic bonding. In a detailed embodiment,the ionomer comprises a co-polymer of ethylene and methacrylic acid, oracrylic acid, which has been neutralized with the addition of a saltwhich supplies a cation such as Li+, Na+, K+, Ca++, Zn++, Mg++, Al+++,or a co-polymer of ethylene and a vinyl ester (i.e., ethylenemethylmethacrylate co-polymer to which cations such as those listedabove have been added by saponification of the ester.

Adding 14% comonomer of octene to the metallocene polyethylene producesan inner layer 12 having excellent optical clarity. Moreover, the innerlayer 14 has improved physical properties because the catalyst methodused to prepare the material produces a polymer with a narrow molecularweight distribution. Polymers made with the usual catalysts tend to havesignificant components of lower molecular weights. The latter reduce themechanical properties of the overall polymer, as compared with thehigher molecular weight components of the polymer. Because a polymermade from a metallocene catalyst has a tighter molecular weightdistribution and fewer lower molecular weight components, it exhibitsgreater mechanical strength and puncture resistance.

Although metallocene polyethylene has good optical properties, it bondsadhesively (no cohesive failures to most materials). Metallocenepolyethylene is also difficult to process due to its narrow molecularweight distribution. A thermoplastic material with a narrow molecularweight distribution has a narrow melting range, making fabrication of asheet of the material or lamination difficult. Providing outer layers14, 16 formed of ionomer solves these problems.

In one embodiment, the outer layers 14, 16 are formed of an ionomerhaving a high acid content (i.e., at least 4% (by weight) free acid).The high acid content provides strong bonds (i.e., cohesive failures ondelamination testing) and improves the optical properties of theionomer. The metallocene polyethylene inner layer 14 comprises ethylenealpha-olefin with 14% comonomer of octene. This three layered structuredisplays interesting optical properties. When this encapsulant materialis laminated under heat and pressure, it appears cloudy and light blue.However, when the total light transmission through the material ismeasured using an integrating sphere, it has been found that the totalamount of light going through the material is over 90%. This is due tomicro or nano crystalline size regions within the outer ionomer layerswhich scatter the incident light. The result of encapsulating a solarcell with the encapsulant of the invention is that there is no reductionin light reaching the cell as compared to an EVA encapsulated solarcell.

The following table indicates this result. The short circuit currentdensity (J_(sc) in mA/cm²), which is a direct measure of the amount oflight reaching the solar cell, was measured on solar cells withoutlamination and solar cells laminated with the encapsulant of theinvention and with EVA under a piece of glass. The results indicatetransmission at least as good as EVA and possibly even better.

J_(sc)-post Sample # Encapsulant J_(sc)-bare cell lamination Δ (%) 1 EVA28.23 26.89 −4.7 2 EVA 28.52 27.20 −4.6 4 Invention 28.55 27.45 −3.8 5Invention 28.46 27.36 −3.9 6 Invention 27.02 26.20 −3.0 7 Invention27.30 26.43 −3.2

The encapsulant material 10 can be formed by any number of film or sheetcoextrusion processes, such as blown-film, modified blown-film,calendaring, and casting. In one embodiment, the encapsulant material 10is formed by coextruding, in a blown film process, the metallocenepolyethylene layer 12 and the ionomer layers 14, 16. In particular, thelayer of metallocene polyethylene 12 includes first and second sublayers12 a, 12 b of metallocene polyethylene. The first ionomer layer 14 iscoextruded with the first sublayer 12 a of metallocene polyethylene, andthe second ionomer layer 16 is coextruded with the second sublayer 12 bof metallocene polyethylene. The first layer 12 a of metallocenepolyethylene (along with the first ionomer layer 14) is then bonded tothe second layer 12 b of metallocene polyethylene (along with the secondionomer layer 16) to produce the encapsulant material 10. In this way, athicker encapsulant layer and the desired 3-layer structure can beformed.

The ionomer layers 14, 16 can have a thickness in the range of0.001-0.004 inch, and the layer 12 can be of any desired thickness. Forsolar cell applications, the layer 12 can have a thickness ofapproximately 0.015 inch such that the overall thickness of theencapsulant material 10 has a thickness of approximately 0.018 inch. Theencapsulant material 10 can be manufactured as elongated sheet that canbe stored in convenient rolls of any desired width.

FIG. 2 is a cross-sectional view of a solar cell module 20 in which theencapsulant material 10 encapsulates interconnected crystalline siliconsolar cells 22. The encapsulant material 10 is disposed adjacent thefront 23 and rear surfaces 24 of the interconnected solar cells 22. Theencapsulant material 10 adjacent the rear surfaces 24 of theinterconnected solar cells 22 may be pigmented. The encapsulant material10 may be bonded to the interconnected solar cells 22. A front supportlayer 26 formed of light transmitting material covers front surfaces 23of the encapsulated interconnected solar cells 22. The front supportlayer 26 may be formed of glass or transparent polymer. A backskin layer28 is disposed adjacent to the rear surfaces 24 of the encapsulatedinterconnected solar cells 22. The backskin layer 28 can be formed of(1) a polymer such as tedlar laminate, (2) a thermoplastic material thatcan be used to form edge sealing, thus eliminating the need for analuminum frame, or (3) a piece of glass forming a double glass module.In one detailed embodiment, the backskin layer 28 can be a thermoplasticpolyolefin comprising a mixture of at least two ionomers such as asodium ionomer and a zinc ionomer with or without 10-20% (by weight)glass fiber.

FIG. 3 is a cross-sectional view of a solar cell module 30 in which theencapsulant material 10 and the backskin layer 28, in combination,encapsulate the interconnected solar cells 22. The encapsulant material10 is disposed adjacent the front surfaces 23 but not the back surfacesof the interconnected solar cells 22. The encapsulant material need notbe placed adjacent the rear surfaces 24 of the interconnected solarcells 22. The backskin layer 28 serves as the rear encapsulant and asthe rear surface of the module.

FIG. 4 is a cross-sectional view of a solar cell module 40 whichincludes a thin film of Copper Indium Diselenide (CIS). A Zinc Oxide(ZnO) layer 32 is disposed on a front surface 31 of the Copper IndiumDiselenide (CIS) film 34 and a back contact 36 is disposed on a rearsurface 33 of the film 34. The encapsulant material 10 is disposed onthe ZnO layer 32 and the front support layer 26 is disposed on theencapsulant material 10. The substrate layer 28, which can be formed ofglass, plastic or metal, is disposed adjacent a rear surface of the backcontact 36. For purposes of this invention, CIS is considered equivalentto the general class of I-III-VI₂ compounds such as the pentenarycompound Cu(In,Ga)(Se,S)₂. Also, the transparent conducting layer (i.e.,the ZnO layer) is considered the equivalent to the combination of thetransparent conducting layer with a thin buffer layer (e.g., a 500 Ålayer of CdS).

FIG. 5 shows a cross-sectional view of an amorphous silicon solar cellmodule 50 comprising the encapsulant material 10. A layer of thintransparent conducting oxide 42 (e.g., Tin Oxide (SnO₂)) is coated on afront support layer 26 comprising glass. An amorphous silicon layer 44is disposed adjacent a rear surface of the oxide layer 42, and a rearcontact 46 is disposed adjacent a rear surface of the amorphous siliconlayer 44. The encapsulant material 10 is disposed adjacent a rearsurface of the rear contact 46. The backskin layer 28, which can beformed of glass, plastic or metal, is disposed adjacent a rear surfaceof the encapsulant material 10. A front support layer comprising glassis disposed on the oxide layer 42.

FIG. 6 is a cross-sectional view of a Cadmium Telluride (CdTe) thin filmmodule 60. A rear surface of the front support layer 26 is coated with athin transparent conducting oxide layer 42. A layer of Cadmium Sulfide(CdS) 52 is placed adjacent to a rear surface of the oxide layer 42, anda layer of Cadmium Telluride (CdTe) 54 is placed adjacent a rear surfaceof the CdS layer 52. A rear contact 42 is placed adjacent a rear surfaceof the CdTe layer 54. The encapsulant material 10 is placed adjacent arear surface of the rear contact 42, and a backskin layer 28 is disposedadjacent a rear surface of the encapsulant material 10.

FIG. 7 is a cross-sectional view of a laminated glass or transparentpolymer assembly 70. A front support layer 62 formed of transparentmaterial is disposed adjacent a front surface 11 of the encapsulantmaterial 10 and a rear support layer 64 also formed of transparentmaterial is disposed adjacent a rear surface 13 to the encapsulantmaterial 10. The entire assembly 70 is then laminated to encapsulate thesupport layers 62, 64 with the encapsulant material 10.

The invention also features a UV stabilization additive package employedin the encapsulant material to prevent degradation. Photo-oxidation(oxidation caused by UV light) and thermal-oxidation (oxidation causedby heat) are two mechanism that cause degradation. By including astabilization additive package, the encapsulant material has the abilityto withstand degradation for an extended period of time. When used insolar cell modules, the encapsulant material has the capability towithstand degradation for up to a thirty year service life. When used inlaminated glass or transparent polymer applications, the encapsulantmaterial can last even longer, since the encapsulant has no exposure tooxygen or water vapors except at the edges of the laminated structure.If the edges are well sealed, the likelihood of any photo-oxidationtaking place is very low.

The stabilization additive package must be appropriate for theapplications in which the encapsulant material is to be used and mustsatisfy solubility limitations of the two materials used as themetallocene polyethylene layer 12 and the ionomer layers 14, 16 of theencapsulant material 10. A stabilization additive package has to besoluble in both materials up to the level desired. Otherwise, aconcentration gradient and migration would occur. Metallocene polymersgenerally have lower solubility than ionomers, making the selection ofstabilizers a non-trivial matter.

It has been determined that the stabilization additive package of theinvention need not contain an ultraviolet light absorber (UVA) nor aphenolic anti-oxidant (AO). Since in both solar cell module andlaminated glass applications, the glass filters a significant amount ofthe ultraviolet light from the sun, an ultraviolet light absorber is notneeded. Furthermore, some ultraviolet light absorbers are known toresult in yellowing themselves. It has also been determined that thestabilization system need not contain a phenolic AO, because theencapsulant material shows no significant loss of mechanical propertiesafter repeated extrusions and again, phenolic AOs are known to result inyellowing themselves.

Based on the foregoing considerations, the ultraviolet stabilizationadditive package of the invention comprises a combination of hinderedamine stabilizers. One hindered amine light stabilizer provides thermaloxidative and photo-oxidative stabilization and the second hinderedamine light stabilizer provides mainly photo-oxidative stabilization.

In one embodiment, the stabilization additive package includes 0.1-0.25%hindered amine with a high order of protection against thermal oxidationand photo-oxidation and 0.25-1.0% of hindered amine with a high order ofprotection mainly against photo-oxidation. Ideally, one hindered aminewould be preferred for both activities. However, a hindered amine thatcan perform both functions must also be sufficiently soluble inmetallocene polyethylene and ionomer used in the encapsulant material,making this search difficult.

Examples of hindered amine stabilizers that provide thermal oxidativestabilization as well as photo-oxidative stabilization are1,3,5-Triazine-2,4,6-triamine,N,N′″-[1,2-ethanediylbis[[[4,6-bis[butyl(1,2,2,6,6-pentamethyl-4-piperidiny)amino]-1,3,5-triazine-2-yl]imino]-3,1-propanediyl]]-bis[N′,N″-dibutyl-N′,N″-bis(1,2,2,6,6-pentamethyl-4-piperidinyl)-(Chimassorb119, CAS Reg. No. 106990-43-6);N,N′-bis(2,2,6,6-Tetramethyl-4-piperidinyl)-1,6-hexandiamine, polymerwith 2,4,6-trichloro-1,3,5-triazine and 2,4,4-trimethyl-1,2-pentamine(Chimassorb 944, ACS Reg. No. 70624-18-9); andN,N′-bis(2,2,6,6-Tetramethyl-4-piperidinyl)-1,6-hexandiamine polymerwith 2,4,6-trichloro-1,3,5-triazine and tetrahydro-1,4-oxazine (CyasorbUV 3346).

Examples of hindered amine stabilizers that provide photo-oxidativestabilization are dimethyl succinate polymer with4-hydroxy-2,2,6,6-tetramethyl-1-piperidine ethanol (Tinuvin 622, CASReg. No. 65447-77-0); bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate(Tinuvin 770, ACS Reg. No. 52829-07-9); propandioic acid,[(4-methoxyphenyl)-methylene], bis (1,2,2,6,6-pentamethyl-4-piperidinyl)ester, (CAS Registry No. 147783-69-5, Sanduvor PR-31);Poly-methylpropyl-3-oxy-[4(2,2,6,6-tetramethyl)piperidinyl] siloxane(Uvasil 299HM); and3-Dodecyl-1-(2,2,6,6-tetramethyl-4-piperidinyl)-2,5-pyrrolidinedione(Cyasorb UV 3604).

In another embodiment, the hindered amine light stabilizers may begrafted onto a polymer structure. The Sanduvor PR-31 represents a newclass of hindered amine light stabilizers which graft onto a polymerstructure. Once a hindered amine light stabilizer is grafted onto apolymer, it remains in place as a stabilizer in the polymer.

Equivalents

While the invention has been particularly shown and described withreference to specific preferred embodiments, it should be understood bythose skilled in the art that various changes in form and detail may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. An encapsulant material consisting essentiallyof: a first ethylene copolymer; a second ethylene copolymer; and anultraviolet light stabilization additive package comprising: a firsthindered amine light stabilizer providing thermal oxidativestabilization and photo-oxidative stabilization, wherein the firsthindered amine light stabilizer is soluble in the first ethylenecopolymer and the second ethylene copolymer and is selected from a groupconsisting essentially of1,3,5-Triazine-2,4,6-triamine,N,N′″-[1,2-ethanedinylbis[[[4,6-bis[butyl(1,2,2,6,6-pentamethyl-4-piperidinyl)amino]-1,3,5-triazine-2-yl]amino]-3,1-propanedinyl]]-bis[N′,N″-dibutyl-N′,N″-bis(1,2,2,6,6-pentamethyl-4-piperidinyl)-;N,N′-bis(2,2,6,6-Tetramethyl-4-piperidinyl)-1,6-hexandiamine, polymerwith 2,4,6-trichloro-1,3,5-triazine and 2,4,4-trimethyl-1,2-pentamine;and N,N′-bis(2,2,6,6-Tetramethyl-4-piperidinyl)-1,6-hexandiamine polymerwith 2,4,6-trichloro-1,3,5-triazine and tetrahydro-1,4-oxazine; and asecond hindered amine light stabilizer providing photo-oxidativestabilization, wherein the second hindered amine light stabilizer issoluble in the first ethylene copolymer and the second ethylenecopolymer and is selected from a group consisting essentially ofDimethyl succinate polymer with4-hydroxy-2,2,6,6-tetramethyl-1-piperidine ethanol; and propandioicacid, [(4-methoxyphenyl)-methylene]-, bis(1,2,2,6,6-pentamethyl-4-piperidinyl) ester.
 2. The encapsulant materialof claim 1 wherein the first hindered amine light stabilizer forms0.1-0.25% of the encapsulant material.
 3. The encapsulant material ofclaim 1 wherein the second hindered amine light stabilizer forms0.25-1.0% of the encapsulant material.
 4. The encapsulant material ofclaim 1 wherein the second hindered amine stabilizer is capable of beinggrafted to the encapsulant material.
 5. The encapsulant material ofclaim 4 wherein propandioic acid, [(4-methoxyphenyl)-methylene]-, bis(1,2,2,6,6-pentamethyl-4-piperidinyl) ester forms the second hinderedamine stabilizer and is capable of being grafted to a polymerencapsulant material.
 6. The encapsulant material of claim 1 wherein thefirst hindered amine light stabilizer is1,3,5-Triazine-2,4,6-triamine,N,N′″-[1,2-ethanedinylbis[[[4,6-bis[butyl(1,2,2,6,6-pentamethyl-4-piperidinyl)amino]-1,3,5-triazine-2-yl]amino]-3,1-propanedinyl]]-bis[N′,N″-dibutyl-N′,N″-bis(1,2,2,6,6-pentamethyl-4-piperidinyl)-and the second hindered amine light stabilizer is Dimethyl succinatepolymer with 4-hydroxy-2,2,6,6-tetramethyl-1-piperidine ethanol.
 7. Theencapsulant material of claim 1 wherein the first ethylene copolymer isa metallocene polyethylene.
 8. The encapsulant material of claim 1wherein the second ethylene copolymer is a copolymer of ethylene andacrylic acid.
 9. The encapsulant material of claim 1 wherein the secondethylene copolymer is a copolymer of ethylene and a vinyl ester.
 10. Theencapsulant material of claim 9 wherein the copolymer of ethylene and avinyl ester is an ethylene methylmethacrylate copolymer.
 11. Theencapsulant material of claim 1 wherein the second ethylene copolymer isan ionomer.
 12. The encapsulant material of claim 1 wherein theencapsulant material comprises a three layer structure having an innerlayer comprising the first ethylene copolymer and two outer layerscomprising the second ethylene copolymer.